These days, burls are harvested and kept wet. They are taken to the various mills to be cut, and they are cut, graded, and boiled. The tannins and acidic juice in the plant (kind of pinkish) are removed, and it stabilized the wood a great deal (
https://pipedia.org/docs/CharacteristicsOfBriar.pdf).
So you get fresh wood, it's pretty wet. If you make a pipe, it will continue to dry and change shape and dimension, might crack. Better to let it sit. The cutters suggest 2 years at least, and certainly between 2 years and 5 years after milling, it's much more stable, it's harder, what wood workers call "seasoned". At ten years it's harder yet, has a tendency toward burning rather than cutting. And at twenty and thirty years on, it hasn't changed much. Like most aging processes, there's diminishing returns the further up the curve you go.
What's happening? Oxydation of lignin or something... I'm not even sure. But the difference bewteen fresh milled briar and aged is easy to detect. The color and the physical properties of the wood change.
All this must be kept separate in one's head from the "age" of the wood, which is usually 30-50 years old when harvested. Younger plants are too small, older ones hard to find, often dead or cracked or attacked by insects.
And that's it. There's no magic needed. It's wood, and it's full of minerals because it grows on crusty junk soil. And it makes good pipes because of that.